Vacuum machines are currently known which have a bell-shaped chamber and comprise a vacuum pump which is connected to a chamber, also known as vacuum chamber, inside which there is usually a preformed pouch, which is open on three sides and inside which a food product is placed.
In these machines of the known type, the preformed pouch, with the product already inside, is then inserted in the vacuum chamber and then vacuum is produced inside the chamber; a heat seal is then provided at the free side of the preformed pouch.
This known technology entails the need to use preformed pouches which therefore have a size that can be compared to that of the product. Moreover, considerable labor is required.
Operating steps therefore occur which increase the time required to achieve vacuum packaging of the product. Furthermore, if packaging occurs in a controlled atmosphere, a large proportion of the gas is wasted, since the gas is dispersed not only into the pouch but mostly also inside the vacuum chamber.
EP603704 is also known in which food products are packaged by using a film of plastic material which is guided through a folding station in order to obtain a double layer and is then heat-sealed so as to define pouches.
These pouches are obtained in the desired size or capacity before being filled and sealed.
Heat-sealing and forming are followed by filling, as well as by the closing and cutting of the pockets or pouches thus obtained. However, this solution is complicated, in that it has multiple and distinct stations for performing the individual operations to which the film of plastic material must be subjected in sequence.
Furthermore, hot or cold forming of the plastic film is provided, and so is the filling, by means of a nozzle, with liquid products, flours or finely granulated products, and therefore it is not possible to consider the use of the described solution for solid products having preset volumes, such as sliced ham and salami, meat, cheese in portions, trays containing food and nonfood products.
Moreover, in the described known solution, in order to achieve closure of the pocket or pouch it is necessary to pass it through various steps of operation, with the consequent need for optimum centering of the pouch at each station so as to allow optimum and desired processing.
EPA0405718 is also known which illustrates a solution for a packaging device, particularly for packaging food products or technical materials that can be inserted between longitudinally folded parts of a single-folded film from one of its open sides. The device comprises thermal bonding means or heat-sealing means which are substantially L-shaped so as to seal the film both at the open longitudinal side and at its transverse sides, and at least one nozzle for generating vacuum and/or for introducing the gas or mixture.
EP0832819B1 is also known which has, as its priority, Italian application AL960002 dated 26 Sep. 1996, which describes a packaging device which comprises a vacuum chamber which can be opened to receive a length of at least one single-folded film with the material to be packaged inserted therein, and respectively can be closed hermetically, after which it is possible to produce vacuum and/or introduce the gas or mixture with the portion or length of film assuming a pocket-like configuration, heat-sealing means being furthermore provided which are arranged for operation within the vacuum chamber, the at least one nozzle being arranged at the vacuum chamber so as to be directed, after its closure, toward the open longitudinal and/or transverse sides of the length of film.
This solution, too, is not free from drawbacks, since the forming of micro-creases has been observed during the heat-sealing of the film, and this can entail problems in tightness and worsens the aesthetic appearance of the package.
Furthermore, the flow of the gas inside the film is not optimum, reducing the efficiency of the device.
Finally, when the operator pulls the sheet, the sheet can assume an incorrect arrangement.
Another problem is also noted: when vacuum is provided in a bell, and accordingly in the pouch being formed, it is then necessary to compensate the vacuum partially with the gas; however, if the compensation is excessive, the bell opens during heat-sealing.
Since one works with a system in a particularly variable equilibrium, there is no precise control over the volume of gas introduced in the bag or pouch being formed.
The quantity, both in absolute terms and in relative terms (%), of gas that remains inside the pouch being formed is therefore extremely variable and difficult to control, and this leads to the provision of a series of pouches or packages which are more or less inflated or deflated depending on the situation.
The problem is then worsened if it is necessary to change package format often.
Another problem is observed in the step of heat-sealing of the film, which is heat-sealed and cut on the transverse side with respect to the direction of advancement of the film: since it is often made of heat-shrinking material, the film can retract, reducing its width, and this might prevent the accommodation of the oncoming tray.
The tray can thus arrive and find a space that is insufficient to accommodate it, and this causes its diversion toward the open side of the pouch, ending on the longitudinal heat-sealing bar, causing machine downtimes and waste of time for cleaning the machine.
Finally, it is noted that the return of the air into the bell or vacuum chamber occurs, due to constructive reasons, usually in a region that is far from the heat-sealing bar or coincides with the vacuum hole by means of the use of a three-way valve, but this does not optimize the conveyance of the flow inside the bell.
Heat-sealing machines for preformed trays are also known. In order to use heat-sealing machines, and therefore when it is necessary to change the size of the package to be sealed and then trim the film heat-sealed onto the edge of the tray with a die cutter, the user must intervene by changing at least one mechanical part, commonly known as die.
Such die is constituted by a hot upper element, which is shaped like the tray to be packaged, by a die-cutter, which will follow the edge of the tray in order to cut the film, and by a lower element, which constitutes the receptacle of the tray for heat-sealing and cutting.
The problem that is observed is constituted by the considerable time required to change the die as a function of the shape of the tray to be heat-sealed.
Machines are also known which, in order to give greater flexibility to the system, provide, once the machine has stopped, an automatic changing of the upper element of the die. Such upper element is provided with two or more (up to four) dies which have two or more vacuum chambers which rotate on a horizontal axis.
In this manner, the operator, in order to change the die, must, after stopping the machine, select which die he or she will use and the die will be preheated.
Once the desired temperature has been reached, it is necessary to rotate the upper element, bringing it at the work region, and replace physically the lower element (which must match up exactly with the upper one) in order to be then able to begin work with the new format.
This solution is mechanically very complicated and expensive, and it also has processing downtimes that affect the overall costs of the machine.